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OPINION: It’s time to restart the nuclear thermal rocket program

File footage of the Phoebus 1B reactor in operation during a test in February 1967. Credit: NASA

File footage of the Phoebus 1B reactor in operation during a test in February 1967. Credit: NASA

Though it sounds like some far-fetched contrivance from the mind of a science fiction writer, nuclear thermal rockets (NTRs) are a very real – and incredibly efficient – means of propulsion under consideration for human missions to Mars. Not only are they real, but they have already been designed and tested…albeit more than 40 years ago.

Unfortunately, amidst a downturn in public support for nuclear-related programs of any sort in the early 1970s, the Nuclear Engine for Rocket Vehicle Application (NERVA) program was cancelled in 1972 and never resumed.

However, with NASA’s Space Launch System (SLS) marching towards its uncrewed maiden launch in 2018, and the agency’s goal of sending a crewed mission to Mars some time in the 2030s, there has been a renewed interest in propulsion systems that may help to get craft and crew to the Red Planet more efficiently, and more quickly, than traditional chemical propulsion systems.

NASA’s Tony Kim thinks that nuclear thermal rockets answer the program’s needs, and he’s ready to get started. Kim, the project manager for nuclear propulsion at NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Alabama, is one of the strongest supporters for nuclear propulsion within the agency and was recently on-hand at an event at NASA’s Michoud Assembly Facility to discuss the program.

Effectively starting with the designs from the shuttered NERVA program, Kim hopes to develop a nuclear thermal engine capable of safely sending a crewed mission to Mars, and returning them to Earth, consuming only liquid hydrogen and 2.5 kilograms (5.5 pounds) of low-enriched fissionable material in the process. He posits that such a mission is not only possible, but resuming the NTR program could eventually yield engines significantly more capable, and more efficient, than those from NERVA.

But why nuclear? Historically, crewed spacecraft have relied on chemical propellants – solid, cryogenic, semi-cryogenic, and hypergolic – to produce the thrust that propels the craft forward. Though this may produce a high amount of thrust, it does so at the cost of efficiency. There is a theoretical limit to the efficiency of a chemical engine, and many engines in-use today are at – or near – that limit. In other words, they’re about as good as they’re going to get.

NERVA nuclear thermal rocket engine diagram. Credit: NASA

NERVA nuclear thermal rocket engine diagram. Credit: NASA

That lower efficiency means that a spacecraft must carry more fuel to achieve the desired final velocity, thus increasing the overall mass of the craft. Nuclear thermal engines, though, are at least twice as efficient as the best chemical rockets and hold the promise to reduce Mars transit times from 8-9 months to 3-4 months. This translates to huge potential upsides to mission planners.

Foremost is the reduced exposure to cosmic radiation astronauts may encounter on their voyage. Free from the protection of Earth’s magnetic field, harmful radiation exposure is a very real concern for both electronic components and living organisms on a deep space flight. Shorter travel times should positively impact crew health and mitigate the risk of spacecraft systems failing from errant high-energy radiation.

Another significant benefit is the reduced load on life support systems. Shorter transits may translate to decreased mass in consumables needed to support the crew. Reduced travel time may also diminish concerns about the mental health of the crew, along with the general ‘wear-and-tear’ on the spacecraft and its systems.

So, why hasn’t NASA been using these engines if they’ve already been tested? In a word: politics. ‘Nuclear’ has been a dirty word with the American public after the incidents at Three Mile Island, Chernobyl, and – more recently – at Fukushima. Politicians had been reluctant to re-start many nuclear-related programs for fear of a public backlash.

Thankfully, the tide of negative public opinion may be turning. There are currently four new nuclear reactors under construction in the United States, the first such projects in 30 years, and may signal a change in both public and political sentiment.

Should politicians muster the the will to resume the nuclear thermal rocket program, it will send a clear signal that our elected officials are serious about the Journey to Mars.

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